1. Field of the Invention
This invention relates to a voltage-dependent nonlinear resistor which is often referred to as a varistor, and more particularly, to a ceramic composition for use in varistors comprising strontium, barium, calcium, and titanium as main ingredients.
2. Prior Art
Varistors are resistor elements whose resistance varies in a nonlinear relationship to a change of applied voltage, and more particularly, resistor elements whose resistance drops abruptly when a voltage in excess of a certain value is applied. Various varistors have been used in electronic equipment in order to accommodate or remove abnormal voltage and noise occurring therein.
For example, varistors based on strontium and titanium have a capacitor function in addition to the nonlinearly changing resistance and are adequate for accommodating or removing abnormal voltage and noise. These varistors, however, suffer from the problem that the varistor voltage at which resistance drops abruptly lowers as the temperature rises. Therefore, the increasing ambient temperature or self heat release can cause the varistors to induce overcurrent or thermal runaway.
One solution is disclosed in JP-A 45559/1991 as a varistor comprising strontium, barium, calcium, and titanium as main ingredients. Since the varistor voltage increases or remains substantially unchanged as the temperature rises, this varistor restrains the potential overcurrent and thermal runaway resulting from the increasing ambient temperature or self heat release. The varistor preparing process involves heat treatment of a sintered semiconductor ceramic for re-oxidation. A varistor exhibiting a desired varistor voltage is then obtained by properly setting the treating temperature. Therefore, varistors having different varistor voltages can be obtained from a common semiconductor ceramic material by carrying out heat treatment at different temperatures.
The performance of varistors is represented by a nonlinear index .alpha.. The nonlinear index .alpha. is generally expressed by EQU .alpha.=1/log(E10/E1)
using E10 and E1. E10 is the voltage developed across a varistor when a current flow of 10 mA is conducted therethrough. E1 is the voltage developed across a varistor when a current flow of 1 mA is conducted therethrough. The above-referred JP-A 45559/1991 is not successful in producing from a common composition varistors having a satisfactory nonlinear index .alpha. over a broad voltage region. In one exemplary situation, varistors are fabricated by controlling heat treating temperature such that a varistor voltage E10 may fall within the range between 2 V and 20 V. A certain semiconductor ceramic material can provide a satisfactory .alpha. when E10 is approximately 2 V, but not when E10 is approximately 20 V. Inversely, another semiconductor ceramic material can provide a satisfactory .alpha. when E10 is approximately 20 V, but not when E10 is approximately 2 V. Therefore, when it is desired to change the E10 value of varistors of JP-A 45559/1991 in the range from 2 V to 20 V by setting the heat treating temperature at different levels, a plurality of different semiconductor ceramic compositions must be furnished. Also, the varistors of JP-A 45559/1991 were difficult to fabricate because E10 is sensitive to a change of heat treating temperature, which suggests that the heat treating temperature must be strictly controlled in order to achieve a target E10 value.